The present invention relates to an electrode assembly for an electric discharge tube, and in particular to an electrode assembly for an electric discharge tube, which is located at each of opposite ends of the electric discharge tube and which is composed of an arc discharge electrode carrying an electron-emissive substance and a glow discharge tube surrounding the arc discharge tube.
The applicant proposed, as disclosed in Japanese Patent Nos. 1834467 and 1854477, an electrode device for a high bright and long life electric discharge tube, which can effect glow discharge even during arc discharge in the electric discharge tube, and which can prevent electron-emissive substance from being dissipated by evaporation. Such an electrode device is composed of a rod-like arc discharge electrode carrying or containing an electron-emissive substance and a cup-like or tubular glow discharge electrode surrounding the rod-like arc discharge electrode, in combination, and is adapted to trap, at the inner wall of the cup-like or tubular glow discharge electrode, microparticles of the electron-emissive substance which are evaporated and scattered from the arc discharge electrode due to bombardment by ions generated and floating in the electric discharge tube, thereby it is possible to reuse the electron-emissive substance trapped at the inner wall so as to prolong the use life of the electric discharge tube and to prevent the wall surface of the electric discharge tube from blackening due to sticking of the electron-emissive substance.
However, the above-mentioned electrode device offers such a disadvantage that it cannot be easily set up in a thin electric discharge tube formed of a small diameter tube body since it has such a configuration that the rod-like arc discharge electrode is laid by its length in a direction orthogonal to the longitudinal axis of the tube body, and is surrounded by the cup-like glow discharge electrode. Further, it is practically required that a slight gap (of about 0.15 mm) is defined between the inner wall of the tube body and the glow discharge tube, and in order to obtain this gap in a thin tube body, the electrode device should have smaller dimensions, resulting in difficulty of manufacturing the electrode device, and increased manufacturing cost, and accordingly, it is impractical. Further, the rod-like arc discharge electrode should be laid along the center axis of the cup-shape glow discharge electrode in order to expect uniform electric discharge, and accordingly, the alignment thereof is very difficult.